[v4,03/11] mm/gup: Applies counting method to monitor gup_pgd_range
diff mbox series

Message ID 20190927234008.11513-4-leonardo@linux.ibm.com
State Changes Requested
Headers show
Series
  • Introduces new count-based method for monitoring lockless pagetable walks
Related show

Commit Message

Leonardo Bras Sept. 27, 2019, 11:40 p.m. UTC
As decribed, gup_pgd_range is a lockless pagetable walk. So, in order to
monitor against THP split/collapse with the couting method, it's necessary
to bound it with {start,end}_lockless_pgtbl_walk.

There are dummy functions, so it is not going to add any overhead on archs
that don't use this method.

Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
---
 mm/gup.c | 8 ++++++++
 1 file changed, 8 insertions(+)

Comments

Kirill A. Shutemov Sept. 30, 2019, 11:09 a.m. UTC | #1
On Fri, Sep 27, 2019 at 08:40:00PM -0300, Leonardo Bras wrote:
> As decribed, gup_pgd_range is a lockless pagetable walk. So, in order to
     ^ typo
Leonardo Bras Sept. 30, 2019, 2:27 p.m. UTC | #2
On Mon, 2019-09-30 at 14:09 +0300, Kirill A. Shutemov wrote:
> On Fri, Sep 27, 2019 at 08:40:00PM -0300, Leonardo Bras wrote:
> > As decribed, gup_pgd_range is a lockless pagetable walk. So, in order to
>      ^ typo
> 
Fixed, thanks!
Leonardo Bras Oct. 1, 2019, 5:56 p.m. UTC | #3
On Mon, 2019-09-30 at 14:51 -0700, John Hubbard wrote:
> On 9/27/19 4:40 PM, Leonardo Bras wrote:
> > As decribed, gup_pgd_range is a lockless pagetable walk. So, in order to
> > monitor against THP split/collapse with the couting method, it's necessary
> 
> s/couting/counting/
> 

Thanks, fixed for v5.

> > to bound it with {start,end}_lockless_pgtbl_walk.
> > 
> > There are dummy functions, so it is not going to add any overhead on archs
> > that don't use this method.
> > 
> > Signed-off-by: Leonardo Bras <leonardo@linux.ibm.com>
> > ---
> >  mm/gup.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 98f13ab37bac..7105c829cf44 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2325,6 +2325,7 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end)
> >  int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> >  			  struct page **pages)
> >  {
> > +	struct mm_struct *mm;
> 
> I don't think that this local variable adds any value, so let's not use it.
> Similar point in a few other patches too.

It avoids 1 deference of current->mm, it's a little performance gain.

> 
> >  	unsigned long len, end;
> >  	unsigned long flags;
> >  	int nr = 0;
> > @@ -2352,9 +2353,12 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> >  
> >  	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
> >  	    gup_fast_permitted(start, end)) {
> > +		mm = current->mm;
> > +		start_lockless_pgtbl_walk(mm);
> >  		local_irq_save(flags);
> >  		gup_pgd_range(start, end, write ? FOLL_WRITE : 0, pages, &nr);
> >  		local_irq_restore(flags);
> > +		end_lockless_pgtbl_walk(mm);
> >  	}
> >  
> >  	return nr;
> > @@ -2404,6 +2408,7 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> >  			unsigned int gup_flags, struct page **pages)
> >  {
> >  	unsigned long addr, len, end;
> > +	struct mm_struct *mm;
> 
> Same here.
> 
> >  	int nr = 0, ret = 0;
> >  
> >  	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM)))
> > @@ -2421,9 +2426,12 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> >  
> >  	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
> >  	    gup_fast_permitted(start, end)) {
> > +		mm = current->mm;
> > +		start_lockless_pgtbl_walk(mm);
> 
> Minor: I'd like to rename this register_lockless_pgtable_walker().
> 
> >  		local_irq_disable();
> >  		gup_pgd_range(addr, end, gup_flags, pages, &nr);
> >  		local_irq_enable();
> > +		end_lockless_pgtbl_walk(mm);
> 
> ...and deregister_lockless_pgtable_walker().
> 

I have no problem changing the name, but I don't register/deregister
are good terms for this. 

I would rather use start/finish, begin/end, and so on. Register sounds
like something more complicated than what we are trying to achieve
here. 

> 
> thanks,

Thank you!
Leonardo Bras Oct. 1, 2019, 7:40 p.m. UTC | #4
On Tue, 2019-10-01 at 12:04 -0700, John Hubbard wrote:
> On 10/1/19 10:56 AM, Leonardo Bras wrote:
> > On Mon, 2019-09-30 at 14:51 -0700, John Hubbard wrote:
> > > On 9/27/19 4:40 PM, Leonardo Bras wrote:
> ...
> > > > diff --git a/mm/gup.c b/mm/gup.c
> > > > index 98f13ab37bac..7105c829cf44 100644
> > > > --- a/mm/gup.c
> > > > +++ b/mm/gup.c
> > > > @@ -2325,6 +2325,7 @@ static bool gup_fast_permitted(unsigned long start, unsigned long end)
> > > >  int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> > > >  			  struct page **pages)
> > > >  {
> > > > +	struct mm_struct *mm;
> > > 
> > > I don't think that this local variable adds any value, so let's not use it.
> > > Similar point in a few other patches too.
> > 
> > It avoids 1 deference of current->mm, it's a little performance gain.
> > 
> 
> No, it isn't. :) 
> 
> Longer answer: at this level (by which I mean, "wrote the C code, haven't looked
> at the generated asm yet, and haven't done a direct perf test yet"), none of us
> C programmers are entitled to imagine that we can second guess both the compiler 
> and the CPU well enough to claim that  declaring a local pointer variable on the
> stack will even *affect* performance, much less know which way it will go!
> 

I did this based on how costly can be 'current', and I could notice
reduction in assembly size most of the time. (powerpc)
But I get what you mean, maybe the (possible) performance gain don't
worth the extra work.

> The compiler at -O2 will *absolutely* optimize away any local variables that
> it doesn't need.
> 
> And that leads to how kernel programmers routinely decide about that kind of 
> variable: "does the variable's added clarity compensate for the extra visual 
> noise and for the need to manage the variable?"

That's a good way to decide it. :)

> 
> Here, and in most (all?) other points in the patchset where you've added an
> mm local variable, the answer is no.
> 

Well, IMHO it's cleaner that way. But I get that other people may
disagree. 

> 
> ...	start_lockless_pgtbl_walk(mm);
> > > Minor: I'd like to rename this register_lockless_pgtable_walker().
> > > 
> > > >  		local_irq_disable();
> > > >  		gup_pgd_range(addr, end, gup_flags, pages, &nr);
> > > >  		local_irq_enable();
> > > > +		end_lockless_pgtbl_walk(mm);
> > > 
> > > ...and deregister_lockless_pgtable_walker().
> > > 
> > 
> > I have no problem changing the name, but I don't register/deregister
> > are good terms for this. 
> > 
> > I would rather use start/finish, begin/end, and so on. Register sounds
> > like something more complicated than what we are trying to achieve
> > here. 
> > 
> 
> OK, well, I don't want to bikeshed on naming more than I usually do, and 
> what you have is reasonable, so I'll leave that alone. :)
> 
> thanks,

Thank for the feedback,

Patch
diff mbox series

diff --git a/mm/gup.c b/mm/gup.c
index 98f13ab37bac..7105c829cf44 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2325,6 +2325,7 @@  static bool gup_fast_permitted(unsigned long start, unsigned long end)
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages)
 {
+	struct mm_struct *mm;
 	unsigned long len, end;
 	unsigned long flags;
 	int nr = 0;
@@ -2352,9 +2353,12 @@  int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 
 	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
 	    gup_fast_permitted(start, end)) {
+		mm = current->mm;
+		start_lockless_pgtbl_walk(mm);
 		local_irq_save(flags);
 		gup_pgd_range(start, end, write ? FOLL_WRITE : 0, pages, &nr);
 		local_irq_restore(flags);
+		end_lockless_pgtbl_walk(mm);
 	}
 
 	return nr;
@@ -2404,6 +2408,7 @@  int get_user_pages_fast(unsigned long start, int nr_pages,
 			unsigned int gup_flags, struct page **pages)
 {
 	unsigned long addr, len, end;
+	struct mm_struct *mm;
 	int nr = 0, ret = 0;
 
 	if (WARN_ON_ONCE(gup_flags & ~(FOLL_WRITE | FOLL_LONGTERM)))
@@ -2421,9 +2426,12 @@  int get_user_pages_fast(unsigned long start, int nr_pages,
 
 	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) &&
 	    gup_fast_permitted(start, end)) {
+		mm = current->mm;
+		start_lockless_pgtbl_walk(mm);
 		local_irq_disable();
 		gup_pgd_range(addr, end, gup_flags, pages, &nr);
 		local_irq_enable();
+		end_lockless_pgtbl_walk(mm);
 		ret = nr;
 	}